Transportation activities employing internal combustion engines continue to represent an increasingly important aspect of everyday life. As a result, there is a constant demand for more efficient engines. This need has become even more important in terms of current worldwide environmental concerns, especially in light of growing emphasis on climate change. To some extent, with technological and engine design advancements, the internal combustion industry has been able to achieve increased performance, better fuel economy, and lower emissions. But, in many cases, these advances have required engines to run under far more severe conditions, such as higher speeds, higher working temperatures, and higher mechanical stresses. In turn, these more extreme conditions have created demands within the lubrication industry to develop new lubricants having improved properties and performance.
Engine oil is vitally important to the protection of engine parts from wear, which directly affects the engine's durability. Most if not all current engine oils contain additives intended to provide a variety of improvements to the performance of the oil. For example, a typical additive package may contain different chemical species each selected to provide a specific task. For example, detergents serve to keep the metal surfaces in the engine free from sludge contaminants. Dispersants keep particle impurities (such as dirt or soot) away from the metal surfaces to prevent them from causing damage to the metal. Anti-acids generally neutralize acids produced in the engine environment that would otherwise cause corrosion in the engine. Viscosity modifiers help the engine maintain an acceptable viscosity across a range of temperatures and conditions.
Important anti-wear additives include zinc dialkyl dithiophosphates (ZDDPs). Despite a very wide range of experimental studies to identify improved anti-wear additives, the lubrication industry has remained dominated by ZDDP. Many scientific studies have been devoted to elucidation of the mechanism by which ZDDP functions as an anti-wear agent. It is currently believed that, at high temperatures and stresses, the ZDDP decomposes, and its subsequent reaction with steel surfaces leads to formation of an amorphous film which consists of zinc and iron poly phosphates, sulfates and sulfides. Extensive characterization of these films, including via x-ray absorption near edge structure spectroscopy (XANES), has revealed that the outer layer of the amorphous films formed by the decomposition of ZDDPs is composed primarily of long chain polyphosphates, whereas inner layers are made up of short chain polyphosphates.
Unfortunately, decomposition of ZDDP in the engine also generates volatile phosphorus species and ash in the engine oils. Furthermore, at the downstream end, the use of ZDDPs damages the catalytic converters in the exhaust system. Accordingly, regulations have been implemented to restrict the amount of ZDDP incorporated into engine oils. Therefore, a need exists for improved lubricants that provide excellent wear protection while alleviating environmental concerns associated with the use of relatively high levels of ZDDP.